Answer: The heart…the greatest thing that will rectify the heart is belief in Allah by remembering Him,

“Verily, in the remembrance of Allah do the hearts find rest.” (Quran 13:28)

Recitation of the Quran, sitting with the righteous, praying at night, preserving the obligatory actions, these things will rectify the hearts. All types of obedience rectify the heart. Just like disobedience corrupts the heart, Na’am.